The present invention relates to power amplifiers in general, and, more particularly, to high-efficiency audio power amplifiers, especially for battery-operated applications.
A major problem associated with power amplifiers is inefficient use of electrical energy. Especially for audio applications, and more specifically in battery-operated portable audio applications, improving the efficiency of the power amplifier has major benefits in terms of performance as well as cost.
Most of the inefficiency of power amplifiers is a result of power dissipated within the power stage. The dissipated power is a function of the difference between the supply voltage and the output voltage of the power stage. In applications such as audio, where the peak-to-RMS ratio is high (about 12 dB), the peak dictates the supply voltage, but, most of the time the output voltage is significantly lower, and thus a significant amount of power is dissipated in the power stage.
Power dissipation (typical to class-A, AB, B, C, D, and Pulse Width Modulation (PWM), and/or output-of-band noise energy (typical to class D and PWM), is the main cause of inefficiency in prior art power amplifiers, and results in excessive electrical power consumption. The heat developed in the power stage must be dissipated, and the need to provide for adequate heat removal impacts the design and performance of integrated circuit components, and requires design compromises and special engineering expertise.
Further limiting factors associated with prior art power amplifies include limitations in their dynamic range (limited by the power-supply voltage), and their inability to achieve output over the full supply voltage range (xe2x80x9crail-to-railxe2x80x9d operation). Moreover, additional major problems in the design of existing power amplifiers include non-linearity and noise.
When designing a power amplifier, these factorsxe2x80x94efficiency, linearity, dynamic range, and freedom from noisexe2x80x94conflict with one another, and optimizing the design to overcome one will compromise the design""s ability to overcome the others.
It is already known in the art that by employing a tracking power-supply which minimizes the difference between the power-supply voltage provided to the power stage and the required output voltage of the power stage, that the dissipated power may be minimized. The minimizing action of a track power-supply is herein denoted by the terms xe2x80x9ctrackxe2x80x9d and xe2x80x9ctrackingxe2x80x9d, and is effected by providing a target function for determining the output of the tracking power-supply. The arguments of the target function may include the input signal to the power amplifier as well as the internal input to the power stage. There are, however, difficulties in implementing a tracking power-supply that is in itself efficient and suitable for a given application. For example, in the prior art are known tracking power-supplies which are based on switched L-C circuits. Because L-C circuits are reactive and store rather than dissipate energy, such tracking power-supplies are efficient. Unfortunately, the inductors of L-C circuits are not suitable for use with integrated circuits, and therefore such prior-art tracking power-supplies are not useful in applications involving miniaturized and/or battery-operated equipment.
There is thus a widely recognized need for, and it would be highly advantageous to have, a high-efficiency power amplifier with linear response, low noise, and with a wide dynamic range. These goals are met by the present invention.
[1] EP0998795, WO9905806 xe2x80x9cMethod and apparatus for performance improvement by qualifying pulses in an oversampled, noise-shaping signal processxe2x80x9d
[2] EP0906659, WO09749175 xe2x80x9cOversampled, noise-shaping, mixed-signal processorxe2x80x9d
[3] xe2x80x9cRelationships between Noise Shaping and Nested Differentiating Feedback Loopsxe2x80x9d, by J. Vanderkooy, and M. O. J. Hawksford, Journal of the Audio Engineering Society, Vol. 47, No. 12, December 1999.
Tracking Power-Supplyxe2x80x94A power-supply capable of providing a variable output voltage to suit the needs of a power amplifier. According to the present invention, an efficient tracking power-supply is implemented, having control logic controlling a network of switched capacitors. By controlling the switches, different network connections can be made, giving rise to different electrical circuits. This allows creating multiple supply voltages with high efficiency at the load terminals, and monitoring voltages through the sensor terminals.
Multi-Level Quantizerxe2x80x94The above tracking power-supply can be viewed as a quantizer (a xe2x80x9cmulti-level quantizerxe2x80x9d) with multiple output levels possible during different time intervals, where the level changes during each time interval according to the voltages on the capacitors.
Network of Switched Capacitorsxe2x80x94the network of switches and capacitors used in the tracking power-supply.
Network Connectionxe2x80x94This is a specific set of connections, created by controlling the switches of the network of switched capacitors. This set of network connections creates an electrical circuit involving some or all of the capacitors, supplies, load terminals and sensor terminals.
Network Statexe2x80x94The state of the network of switched capacitors at a certain time. The voltages across the capacitors define the network state.
I-Bit Statexe2x80x94a specific case of a network state where a 1-bit state per capacitor indicates whether the voltage over it is higher or lower than some target voltage. This is useful when implementing the target capacitors selection algorithm.
Sensorxe2x80x94A sensor is any means of monitoring the network state while causing minimal affect. To monitor voltage over a certain capacitor, an appropriate network connection can be made by the control logic. A sensor for the 1-bit state can be the output of a comparator, comparing the voltage over the capacitor to the target voltage.
Estimated Network Statexe2x80x94An estimated network state is a network state where some or all of the capacitor voltages are estimated rather than directly monitored.
Network Parametersxe2x80x94The network parameters include sufficient information about components involved in the work of switched capacitors. By way of example, this information may include electrical parameters of the load and main supplies, the capacitance of each capacitor, and the time intervals, whether absolute or relative. In certain embodiments of this invention, the control logic may need to known network parameters in order to estimates, or predict, the estimated network state when direct monitoring is not feasible. The network parameters may be supplied to the control logic, or may be measured by the control logic through the sensor, whether during initialization time, during operation, or both.
Time Intervalxe2x80x94A period of time during which the network connection is held fixed. The duration of such time intervals may be constant or variable, depending on the application.
Load Time Intervalxe2x80x94A time interval during which the network connection involves the tracking power-supply output terminals.
Monitoring Time Intervalxe2x80x94A time interval during which monitoring of the network state can be performed. A monitoring time interval can overlap a load time interval.
Control Logicxe2x80x94Logic controlling the network of switched capacitors via the switches, in order to create a desired network connection. The main task of the control logic is to determine the best network connection involving the load at any time interval. The control logic implements a selection algorithm, and attempts to minimize the value of the target function, while conforming to some other criteria. The control logic may be implemented fully in the digital domain, while monitoring the state of the network of switched capacitors through the sensor. Alternatively, the control logic can be implemented in the analog domain. The control logic unit has one or more inputs and one or more control outputs.
Target Functionxe2x80x94At each load time interval, there is an ideal desired output from the track power-supply. Since in general the tracking power-supply cannot provide this output exactly, the target function is a xe2x80x98costxe2x80x99 function that associates a cost with each possible output from the tracking power-supply during the current load time interval. The control logic uses this function as part of the selection algorithm to determine the best network connection for the current load time interval.
Selection Algorithmxe2x80x94The selection algorithm applied by the control logic tries to minimize the target function, while applying additional considerations as well. Such considerations can be of different natures, including, without limitation, minimizing the number of switching operations taking place, keeping voltages on capacitors within certain ranges, keeping voltages on capacitors close to a target voltage, maintaining certain characteristics of the power stage, and so forth.
Constrained Capacitorsxe2x80x94A selection algorithm according to which each capacitor has a target voltage range, and where the capacitor is not allowed to be connected such that it would charge when the voltage across it is above its target voltage range, and vice versa.
Targeted Capacitorsxe2x80x94A selection algorithm according to which each capacitor has a target voltage, and where the capacitor is not allowed to be connected such that it would charge when the voltage across it is above its target voltage, and vice versa.
Target Errorxe2x80x94The error, in the case of the targeted capacitor selection algorithm, of the actual average voltage supplied by a capacitor during a load time interval relative to that capacitor""s target voltage.
Power Stagexe2x80x94The final stage of the power amplifier. Embodiments of the present invention describe a linear power stage and a discrete power stage.
Linear Power Stagexe2x80x94A power according to the present invention having a linear-analog power stage, where the power-supply is a tracking power-supply. The advantage of this approach is that the PSRR (Power Supply Reduction Ratio) that is an inherent feature of a linear analog power stage, reduces the noise generated by the tracking power-supply at the final output.
Discrete Power Stagexe2x80x94A power amplifier according to the present invention having no analog power stage, where the tracking power-supply is connected directly to the power amplifier output, and acts as a Multi-Level Quantizer. In this approach, the noise-shaping loop handles all noise. No linear-analog power components are used, and this is an advantage in certain cases.
Noise-Shaping Loopxe2x80x94A feedback and filtering network that causes the noise energy (whether non-linear errors correlated with the input, or uncorrelated noise) to reside in frequencies where the noise poses no problem. In the audio case, the power amplifier noise energy is shaped into high, inaudible frequencies. In one embodiment of the present invention, the noise-shaping loop may be implemented entirely in the analog domain, around the power stage. Alternatively, in another embodiment of the present invention, the noise-shaping loop may be implemented entirely in the digital domain before the power stage, based on information supplied by the control logic. In yet another embodiment, the noise-shaping loop may be implemented as a hybrid digital-analog domain using an A-to-D converter to convert analog feedback from the output of the power stage into the digital domain.
The main goal of the present invention is to improve the efficiency performance of power amplifiers, in order to reduce electrical power consumption. Another goal is to reduce the impact of critical factors on the design of power amplifiers, and allow for more tradeoffs regarding different parameters typical of power amplifiers such as dynamic range, signal-to-noise ratio, and harmonic distortion. Another goal is to improve the efficiency of DC-to-DC converters and tracking power supplies.
According to the invention, a non-linear, switching-type tracking power-supply is used to eliminate most of the power dissipation, as well as to increase the supply voltage (and thus the dynamic range) to the power stage by utilizing voltage multiplication techniques. A novel aspect of the present invention is the use of an integral feedback control and noise-shaping unit to correct the switching noise, common mode noise and the non-linearity introduced by such a power-supply. This innovation allows utilizing a tracking power-supply that is easy to design as well as inexpensive to manufacture and use, and which is well-suited for integrated circuitry, but which may otherwise normally exhibit an excessive amount of inherent noise. One embodiment of the present invention uses a linear power stage for which supply voltage is taken from a tracking power supply to significantly decrease power dissipation within the power stage. Another embodiment of the present invention uses a discrete power stage, where the final power amplifier output is taken directly from the tracking power supply. Yet another embodiment of the present invention uses the tracking power supply as a high efficiency DC-to-DC converter.
FIG. 1 is a general block diagram illustrating the basic configuration of a linear power stage power amplifier according to the present invention. A primary power source 102 is the source of DC electrical energy, having a positive output Vdd 102-A and a negative output Vss 102-B, which feed into a tracking power-supply 104. Tracking power-supply 104 receives control input from a noise shaper 106 via a control output 106-B, as well as from an input 106-D and an input 106-E from an output 106-C of the noise shaper 106. Tracking power-supply 104 provides a positive supply voltage V+104-A and a negative supply voltage Vxe2x88x92104-B to a power stage 108, with power output terminals 108-A (L+) and 108-B (Lxe2x88x92), which output an amplification of an input signal 110 at an input 106-A. There is at least one supply voltage involved (which is the voltage furnished to power stage 108), and the illustration herein of a positive supply voltage and a negative supply voltage is as a non-limiting example. In another embodiment, a single supply voltage can be utilized in conjunction with a point at a common or ground potential. Where two distinct supply voltages are utilized (such as a positive supply voltage and a negative supply voltage), the term xe2x80x9csupply voltagexe2x80x9d wherein denotes the voltage difference between these two distinct supply voltages. In another embodiment, the polarity of the positive and negative can also be interchanged in order to also create a negative voltage difference between these two distinct supply voltages. In a similar manner, primary power source 102 is illustrated in this non-limiting example as having positive output Vdd 102-A and negative output Vss 102-B, but it is also possible to output a single source voltage relative to a point at a common or ground potential, and, where there are distinct source voltages for both a positive output Vdd and a negative output Vss, the difference between these two distinct voltages is referred to as the xe2x80x9csource voltagexe2x80x9d. As noted previously, noise shaper 106 provides feedback control of tracking power-supply 104 via an output feedback 122 from a differential amplifier 120 to minimize the affect of noise and non-linearity at output 108-A and 108-B. Also, power stage 108 receives an internal input 106-F from noise shaper 106. The input signals from noise shaper 106 help in predicting the required tracking target function. An output load 112 represents the driven audio load, such as a loudspeaker, and in general may be a combination of resistive and possibly reactive elements. An important criterion of power amplifier operation is how closely the output of the power amplifier matches a specified transform of the input signal. The simplest and generally most-desired such transform is that of a multiplicative constant over a specified frequency range, herein denoted as the xe2x80x9camplificationxe2x80x9d K. The difference between the actual output and the transform is herein denoted as the xe2x80x9cerrorxe2x80x9d of the power amplifier.
A general reference for noise shaping loops in both the analog and digital domains is [3].
As is known in the art, the use of feedback can reduce non-linear behavior of an output circuit by applying a feedback from output to input and invert the non-linear behavior. In addition to reducing non-linearity through the use of feedback, the noise shaper also reduces the uncorrelated audible noise of the output signal. It is known in the art that through the use of an auditory sensitivity filter a noise shaper can shift the frequency spectrum of the uncorrelated noise away from the audio spectrum to higher, substantially non-audible frequencies (the process of xe2x80x9cnoise-shapingxe2x80x9d), so that the noise cannot be heard. In this fashion the noise shaper is able to minimize the audible error of the power amplifier.
Another new aspect of the present invention is the use of a novel switched-capacitor tracking power-supply which does not rely on inductors, and is therefore ideal for integrated-circuit use. Variations in the design of the network of switched-capacitors allow the creation of both positive and negative output voltage to double the dynamic range, and also the creation of an output voltage whose absolute value is higher than that of the power supply voltage to further increase the dynamic range.
FIG. 2 shows a basic switched-capacitor tracking power-supply 104 according to the present invention. A capacitor bank 202 has a number of capacitors, illustrated as capacitors 202-A, 202-B, 202-C, 202-D, and 202-E, for storing electrical energy. Any reasonable number of capacitors may be employed. The capacitors do not have to be of the same value. In one embodiment, as shown in FIG. 2, capacitors 202-A, 202-B, 202-C, 202-D, and 202-E are connected in common on one side to Vss 102-B. A V+ selector 204 is connected individually to the other sides of capacitors 202-A, 202-B, 202-C, 202-D, and 202-E via lines 205-A, 205-B, 205-C, 205-D, and 205-E, respectively. In addition, Vdd 102-A and Vss 102-B are also inputs to V+ selector 204. In this manner, V+ selector 204 can selectively connect Vdd 102-A, Vss 102-B or one side of any one of capacitors 202-A, 202-B, 202-C, 202-D, or 202-E to supply voltage terminal V+ 104-A. Likewise, a Vxe2x88x92 selector 206 is also connected to the other sides of capacitors 202-A, 202-B, 202-C, 202-D, and 202-E via lines 205-A, 205-B, 205-C, 205-D, and 205-E, respectively. In addition, Vdd 102-A and Vss 102-B are also input into Vxe2x88x92 selector 206. Thus, Vxe2x88x92 selector 206 can selectively connected Vdd 102-A, Vss 102-B or one side of any one of capacitors 202-A, 202-B, 202-C, 202-D, or 202-E to supply voltage terminal Vxe2x88x92 104-B. In this specific example the possible voltage that can be generated across V+ and Vxe2x88x92 terminals include: 0, xc2x1(Vddxe2x88x92Vss), xc2x1(Vddxe2x88x92Vss-Cn), xc2x1(Cn), xc2x1(Cn-Cm), where Cn and Cm are the voltages on the respective capacitors. A control logic unit 208 controls V+ selector 204 via a control lines 208-A and Vxe2x88x92 selector 206 via a control line 208-B so that V+ selector 204 and Vxe2x88x92 selector 206 are respectively connected to different input lines (lines 205-A, 205-B, 205-C, 205-D, 205-E, Vdd 102-A, and Vss 102-B) according to the desired output voltages V+ 104-A and Vxe2x88x92 104-B and the voltages available on capacitors 202-A, 202-B, 202-C, 202-D, and 202-E, along with Vdd 102-A, and Vss 102-B. An optional voltage sensor 210 monitors the voltages on lines 205-A, 205-B, 205-C, 205-D, and 205-E, and reports the respective voltages thereon to control unit 208. An optional current sensor 212 monitors a current Iin 216 at voltage Vxe2x88x92 104-B, and reports this current to control logic 208. Because current Iin 216 is equal to a current Iout 214, current sensor 212 could also be located in other positions within the circuit. Note that at any given time, both V+ selector 204 and Vxe2x88x92 selector 206 are characterized by respective states according to the selection. State information is taken into account by control logic 208 to determine the network connection.
As noted, feedback control via noise shaper 106 around power stage 108 (FIG. 1) serves as a control system to reduce non-linearity, as well as a noise-shaping filter to control the distribution of noise energy across the spectrum. This allows for much greater flexibility for optimizing the previously-cited design factors, since the non-linearity and noise generated in the power stage can be corrected by this feedback control and noise-shaping unit.
Therefore, by employing a switched-capacitor tracking power-supply and a noise shaper, the present invention allows the use of low-cost, high-efficiency power components, while still achieving noise free linear power output regardless of noise and non-linearity which might be inherent in the components.
Discrete Power Stage
Another aspect of the invention uses the tracking power supply itself to drive the power amplifier output directly, rather than acting as a power supply for a linear power stage, thus creating a discrete power stage. As is well-known in the art, pulse-width-modulation (PWM) and class-D (digital) power amplifiers are based on power switches that toggle the output between two voltages. In general, there technologies generates an over-sampled 1-bit signal using a variety of noise shaping and pulse-width-modulation techniques, and then use a power switch as the power stage to drive a load with this 1-bit signal. An amplifier operating according to such principles can be viewed as a one-bit quantizer with noise-shaping. The quantizer generates quantization noise with amplitude of 0.5 bit. That is, at least 0.5 of the energy output by the power stage is noise. For audio applications, noise-shaping techniques ensure that the quantization noise resides above the audio spectrum at inaudible frequencies (typically above 20Khz). Such general techniques are herein denoted by the term xe2x80x9cone-bit quantizerxe2x80x9d.
There are three major factors causing inefficiency in such one-bit quantizer systems.
The first factor is the energy of the noise itself. Although this noise is inaudible, it is still produced by the output power stage. This energy can be removed from the output signal by filtering, but unless the filter used is a reactive filter, this noise energy will be dissipated as heat. In systems where other than a reactive filter is used, the efficiency is limited.
In systems where a reactive L-C low-pass filter is used, the energy is not wasted, but rather is recycled by the inductors and capacitors. Unfortunately, L-C networks are relatively bulk and generate significant electromagnetic interference (EMI), and thus are not practical to use in many applications, especially portable audio appliances where efficiency is critical.
The other factors that reduce efficiency in such amplifiers a related to imperfections in the power switch. Parasitic capacitance and finite rise and fall times in the power switch cause loss of energy associated with each toggle of the output voltage from one state to another. This loss of energy is proportional to the voltage difference across the switch at the time of the toggle, to the rise/fall times of the switch and to the switching frequency. Most of the energy loss is due to energy dissipation during the rise/fall period of the switch, when the product of the voltage over the switch and the current through it is not close to zero.
Therefore, the high switching frequency associated with one-bit quantizer systems is another factor leading to inefficiency. Due to the excessive amount of noise energy, a high degree of over-sampling to required, and this leads to a high switching frequency.
The fact that the voltage difference at the time of each toggle is extreme, from Vss to Vdd, further increases the energy losses. Some prior art techniques address this problem by trying to reduce the number of switching transitions that take place, as is disclosed, for example, in reference [1].
According to the present invention, replacing the one-bit quantizer of current technologies with a Multi-Level Quantizer, can significantly reduce the effects of all of the above factors, and thereby dramatically increase the efficiency of power amplifiers based on switching principles.
The energy of quantization noise output from a Multi-Level Quantizer is proportional to one-half the average difference between levels. Depending on the number of levels in the Multi-Level Quantizer, this noise energy can be significantly less than that of a one-bit quantizer. Furthermore, because there is less noise energy in the signal, the amount of over-sampling required to reach the same noise performance is greatly reduced, and thus the switching frequency and energy losses are reduced. Moreover, because the voltage across each switch during the switching transition time is much smaller for a Multi-Level Quantizer compared with a one-bit quantizer, the average energy loss associated with switching is reduced. (Typically, this is the average voltage difference between two adjacent levels, versus the full range of Vdd to Vss).
In order to implement a Multi-Level Quantizer, it is necessary to generate multiple voltage levels. According to the present invention, an efficient way to generate multiple voltages from one power supply is by using the same network of switched capacitors as in the tracking power-supply previously described. The target function for the control logic in this case is simply to produce the level closest to the signal at the input of the Multi-Level Quantizer. Other constraints can be added in the control logic, as will be described below by way of a non-limiting example regarding the Constrained capacitor or targeted capacitors selection algorithms.
Therefore, according to the present invention thee is provided a power amplifier receiving electrical energy from a primary source of electrical energy having a Vss source voltage and a Vdd source voltage, the power amplifier receiving an input signal via input terminals and supplying a power output signal via load terminals, the power amplifier including a network of switched capacitors containing at least one capacitor for storing electrical energy, each of the capacitor having a voltage thereon, wherein the network of switched capacitors is operative to configuring electrical circuits between the load terminals, and wherein the electrical circuits include voltages and components selected from a group containing (a) and Vss source voltage, (b) the Vdd source voltage, and (c) a non-negative number of capacitors of the network of switched capacitors.